I recommend running your Ubuntu LiveCD or from a USB stick and downloading 'Boot-Repair' by following the instructions found here Click Here ..click on 'Recommended Repair' once you've scanned your system, it fixes most problems.

Not to beat a dead horse, but please, everyone should take rahim's experience as a warning. From a Windows perspective, THERE ARE NO OTHER OPERATING SYSTEMS!! When you install Windows (Any version) any existing boot loader will be creamed. As far as I know, only Windows does this. At the very least, 'nix type systems all cooperate very nicely with other OS's. This is why it is always strongly recommended that you install Windows FIRST, then unix/linux/etc. If you don't, then you have to jump through a lot of hoops to reconstruct the 'nix boot loader. The suggestions already given should work for you. If not, there are thousands of HowTo's on the Web that are addressed at solving your exact problem. As long as Windows didn't reformat the entire drive, you can recover from this. It's a pain, but possible. Basically, what has to be done is this:

1) The windows boot loader has to be copied and stored away for use by the linux boot loader.
2) The location of the original linux install has to be located.
3) The linux boot loader must be re-installed and set up to boot linux.
4) The linux boot loader must be configured to "chain" to the Saved copy of the Windows boot loader. Windows cannot be booted directly by the linux boot loader because the Windows boot loader is odd, proprietary and unique to Windows. That's why the Linux loader must chain to a copy of it when you want to boot Windows.

Thankfully, all these steps are automated for you by the various repair tools mentioned by other posters. You can do it manually, but it is really touch&go, risky, a bit hard and would only be recommended in very extreme circumstances.

A worst case scenario would be that none of the tools work. You can always use a linux live CD to locate your files on the linux partition and save them to a removable media (for example a thumb drive or usb disk). Then reinstall linux and let it handle setting up the Dual Boot. Once installed, copy your files back into the new linux install. If Windows creamed your entire linux partition, then I hope you made a backup before installing Windows. If not, I am really sorry this had to happen to you.